feat(phase-8): billing engine + invoice PDF + mark-paid + overdue escalation

Backend:
- app/services/invoice_pdf.py: Jinja2+WeasyPrint PDF generation, saves to
  /app/data/invoices/{id}.pdf, updates Invoice.pdf_path
- app/templates/invoice.html: professional branded A4 invoice template with
  school details, line items table, totals, payment instructions, paid receipt
- routers/billing.py: GET /invoices/{id}/pdf (auto-generate on demand, FileResponse),
  POST /invoices/{id}/mark-paid (payment_method + reference → status=paid),
  POST /trigger-generate-invoices (manual trigger), school_name in invoice list
- tasks/billing.py: fix missing func import in generate_monthly_invoices,
  new billing.generate_invoice_pdf Celery task, auto-send email after
  invoice creation, check_overdue upgraded with 7-day warning emails and
  30-day school suspension + suspension email

Frontend:
- BillingPage.vue: full rewrite — status filter tabs, school names (not UUIDs),
  PDF download button, mail icon, Mark Paid modal with method/reference fields,
  overdue rows highlighted, pagination, Generate Invoices trigger button
- api.ts: markInvoicePaid, downloadInvoicePdf, triggerGenerateInvoices

.paul/STATE.md

@@ -3,16 +3,16 @@
 ## Current Position
 
 Milestone: v1.0 — Foundation & Core Services
-Phase: 7 of 15 (School Admin Portal UI — complete)
-Plan: Phase 7 complete — Phase 8 next
-Status: **Phase 7 applied — ready to begin Phase 8**
-Last activity: 2026-03-16 — Phase 7 complete (portal pages: suspension banner, credit meter, license countdown, SMS chart, delivery stats, top-up request form, ticket filter/pagination, school details)
+Phase: 8 of 15 (Billing Engine + Invoice PDF — complete)
+Plan: Phase 8 complete — Phase 9 next
+Status: **Phase 8 applied — ready to begin Phase 9**
+Last activity: 2026-03-16 — Phase 8 complete (invoice PDF via Jinja2+WeasyPrint, mark-paid modal, overdue escalation with 7d warning + 30d suspension, bug fix in billing task)
 
 ## Loop Position
 
 ```
 PLAN ──▶ APPLY ──▶ UNIFY
-  ·        ·        ·     [No active plan — Phase 8 planning next]
+  ·        ·        ·     [No active plan — Phase 9 planning next]
 ```
 
 ## Progress
@@ -26,7 +26,7 @@ PLAN ──▶ APPLY ──▶ UNIFY
 - Phase 5  (On-Prem SMS Polling Agent):      [██████████] 100% ✓
 - Phase 6  (Super Admin Dashboard UI):       [██████████] 100% ✓
 - Phase 7  (School Admin Portal UI):         [██████████] 100% ✓
-- Phase 8  (Billing Engine + Invoice PDF):   [░░░░░░░░░░]   0%
+- Phase 8  (Billing Engine + Invoice PDF):   [██████████] 100% ✓
 - Phase 9  (Email Dispatcher):               [░░░░░░░░░░]   0%
 - Phase 10 (Support Ticket System):          [░░░░░░░░░░]   0%
 - Phase 11 (Monthly Report Generation):      [░░░░░░░░░░]   0%
@@ -37,8 +37,8 @@ PLAN ──▶ APPLY ──▶ UNIFY
 
 ## Next Action
 
-Run: `/paul:plan` for Phase 8 — Billing Engine + Invoice PDF
-Resume file: .paul/ROADMAP.md → Phase 8
+Run: `/paul:plan` for Phase 9 — Email Dispatcher
+Resume file: .paul/ROADMAP.md → Phase 9
 
 ## Repo